Re: Is it worth swapping mags on epi "57 classic" pups
They're just not made with the same quality of materials or workmanship as American or European-made PU's. No getting around that. Don't confuse them with Gibson '57 Classics, which are made to much higher standards. They produce a PU that transmits sound, but aren't as concerned about consistency or tone quality. Not their fault, it's not a major factor at that price point, and they're filling orders as they're spec'd out.
You can improve the sound by removing as much wax as possible (they're packed full of the stuff) and maybe a mag swap too. In that case, they'll do in a pinch until you can afford better PU's. But, remember you can get used high-quality PU's online for about half price. I've replaced all the Asian-made PU's in my guitars, as they just don't have the clarity, depth, and tone quality. If you play with tons of distortion and effects, thru a solid state amp, you may not hear much difference between a good PU and a cheap one, but the better the amp, the more the difference in tone quality becomes apparent.
In my experience, and I am known as the 'King of Mid-priced Imports', Asian-made single coils sound better than Asian-made HB's, presumably because there's less for them to do wrong. Plus P-90's have 2 magents per coil, whereas HB's have 1/2 magnet per coil, so the windings don't have as much impact on the tone. Even so, you'll hear a difference with a high-quality P-90 vs an Asian-made P-90.
I think eventually all medium-priced imports will have well-made, good-sounding PU's, as the market will continue to be more competitive. It's not rocket science to make a PU, and eventually the Asians will learn the 'secrets' of winding excellent PU's, and they'll be able to do it at a much lower cost. That day is coming. They manufacture almost everything else these days. All they have to do is make the investment to hire some experienced PU winders from a big maker or a boutique company, and step up their quality control, and they will be a force to be reckoned with.